The technical field of this invention is medical sonography and, in particular, methods and devices for employing ultrasonic measurements of blood flow to detect and assess diseases or physiological abnormalities.
Inside the brain is a ventricular system which contains and conducts cerebrospinal fluid. This cerebrospinal fluid flows through several ventricles within the brain before being absorbed back into the blood. When drainage is blocked, the buildup of fluid results in a pressure which swells the ventricles and increases the pressure throughout the brain. This causes a condition referred to as "hydrocephalus."
Although the intracranial pressure increase associated with hydrocephalus can be relieved surgically by providing a shunt from the ventricle to the peritoneum, this too can frequently result in complications. Accordingly, it is desirable to provide a method and apparatus for reliably measuring the intracranial pressure.
Intracranial pressure can be measured directly by surgically inserting a pressure transducer inside the cranium. However, the inconvenience of surgery and the necessity of penetrating the skull make this method undesirable.
Indirect evidence of intracranial pressure can be obtained by observing the effect of elevated pressure on the structures inside the cranium. For example, one can ultrasonically measure the size of the ventricle and determine whether it is unusually large or whether it has increased in size.
However, a hemorrhage, either inside the ventricle or in adjacent parenchymal tissue, can obscure the ventricle and make inferences about pressure from observation of its size difficult. Since hydrocephalus is a frequent complication of intracranial hemorrhage in premature infants, this difficulty is frequently encountered in practice.
An additional disadvantage of the above method is that detectable enlargement of the ventricle requires that the hydrocephalus be chronic. Thus, by the time the elevated intracranial pressure is detected, some damage may have already occurred to the intracranial structures.
The difficulty in obtaining the intracranial pressure, as described above, is a manifestation of the more general difficulty of measuring pressure at inaccessible locations within the human body. The process of measuring pressure in, for example, a blood vessel, involves the application of a mechanical pressure to the vessel itself. Typically, an inflatable cuff is placed around an arm and inflated until it cuts off the blood flow. The cuff pressure required to stop the blood flow provides a measure of the pressure driving that flow.
The foregoing method of applying a mechanical pressure to a blood vessel is not well suited for the measurement of pressure in specific blood vessels. For example, in a diabetic patient afflicted with papilledema, it is often desirable to measure the blood pressure in the capillaries leading to the eye. The use of the conventional inflatable cuff to measure blood pressure within the capillaries leading to the retina is made difficult by the lack of a suitable site at which to apply the inflatable cuff.
In some cases, even when a suitable pressure application site is available, the process of inflating the cuff until it cuts off blood flow may be highly intrusive. For example, measurement of local blood pressure using a cuff in connection with the treatment of impotence is hardly a practical option.
An additional disadvantage of the traditional method of measuring blood pressure is that it is unable to detect the rate of change of blood flow as a function of applied pressure. Using the traditional method, one can readily establish the pressure at which blood flow through a vessel ceases. However, one cannot determine, for example, whether the blood velocity began to decrease precipitously at a particular applied pressure or whether it decreased gradually throughout the process of applying an external pressure.
It is an object of this invention to provide a method and apparatus for measuring pressure at specific sites in the body in a non-invasive manner.
It is a further object of the invention to provide a method and apparatus for determining the effect of an applied pressure or blood flow across a broad range of applied pressures.